Re: incrementing a pointer to an array

utab said:

On Sat, 30 Aug 2008 07:58:05 +0000, Richard Heathfield wrote:

utab said:

On Fri, 29 Aug 2008 23:29:22 -0700, subramanian100in@xxxxxxxxx, India
wrote:

The following portion is from c-faq.com - comp.lang.c FAQ list ·
Question 6.13

int a1[3] = {0, 1, 2};
int a2[2][3] = {{3, 4, 5}, {6, 7, 8}}; int *ip; /*
pointer to int */
int (*ap)[3]; /* pointer to array [3] of int */\

ap = &a1;
The trick is here because you are getting the address of the pointer to
the first element, If I am not mistaken...

if you use sizeof on ap you will see that it is associated to the whole
array...8 bytes.

Wrong. ap is a pointer to the first element of the array, but using
sizeof on ap is by no means required to result in 8, and indeed on my
system it does not. Nor need it result in 3*sizeof(int) - and again, on
my system it does not.

#include <stdio.h>

int main(void)
{
int a1[3] = {0, 1, 2};
int (*ap)[3]; /* pointer to array [3] of int */

Hi,

ap = &a1;

Could you clarify this statement for me? is not ap pointer to whole array
of 3?

Yes, ap is a pointer to an array of three ints, and a1 is an array of three
ints, so it's perfectly legal to point ap at a1.

printf("**ap = %d\n", **ap);
printf("sizeof ap = %d\n", (int)sizeof ap); ap++;
return 0;
}

**ap = 0
sizeof ap = 4


Incrementing this address is undefined.

Why?
not incrementing, dereferencing, sorry

Right - but the OP has made it clear that he already knows dereferencing
such a pointer would be illegal. What he's unclear on is why the FAQ says
that incrementing it (just *one* place) is illegal.

Now that I've looked at the FAQ question in question, it seems to me that
this is simply a misunderstanding, and that the FAQ is at fault, but only
mildly so. What Steve is saying is that if your intent is to walk through
the array's objects, getting a pointer to the array isn't the way to do
it. Rather, you get a pointer to the first element in the array, and
increment /that/, in a loop - which is quite right, of course. His WRONG
applies to the error of treating a pointer-to-array as if it were a
pointer-to-first-element-of-array.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
.

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